Dynex/Rivett has developed several electrohydraulic control devices over the past several years which employ the same basic design principles as those described herein. These products have been refined through years of field evaluation and laboratory testing into reliable, easily maintained, durable products. However, this same process of continuous field evaluation and laboratory testing has shown the existence of room for considerable improvement, particularly in the area of integrating additional fluid control functions into a single, compact assembly. In particular, in many applications it is necessary for the main spool to have a "float" position whereby the two working parts of a four-way valve are both simultaneously connected to the return line. In these same applications, it is often necessary to totally block the flow from the work ports back to the return line whenever the main spool is in the neutral position. The blocking of flow from the work ports back to the return line is usually accomplished by means of load-holding check valves which prevent a machine member from slowly drifting due to fluid leakage through the annular clearances between the main spool and the valve body.
The "float" position is preferably located at one extreme end of the main spool stroke, and thus it is essential that the main spool consistently achieve full stroke even under adverse conditions such as battery rundown and electrical line voltage drops. Two-stage electrohydraulic proportional valves which employ a means of feedback between the main spool position and the pilot stage generally have great difficulty in achieving a consistent "float" position. This difficulty arises from the fact that the feedback rate between the main spool and the pilot stage is typically constant throughout the total range of main spool movement. Thus a full rated electrical signal must be applied to move the main spool to the "float" position, and relatively small input voltage drops may prevent the main spool from achieving a full "float" condition.
In applications requiring both a "float" position and load-holding check, some means must be provided to open the load-holding check valves whenever the main spool is moved to the "float" position so that fluid can freely pass from either or both work ports to the return line. In the past this has been accomplished by using an additional pilot valve, either electrically or manually operated. Typically this additional pilot valve and associated load-holding check valves were separate components in the hydraulic circuit, thus resulting in additional hydraulic and electrical circuitry.
Additionally, it has been found highly desirable to minimize the effect of contaminants in the hydraulic fluid on the operation of the valve.